I have a multimeter, though as a quick aside the mirrors don't hear up either, hence my thought it was upstream of where the large current goes into the circuit?
If the fuse F12 doesn't have 12v on it, we'll head back towards the battery, but at the moment as an easy set of checks we'll look for 12v on the fuse and relay.
What happens is 12v comes into the fusebox, through fuse F12, on to the relay. After the relay, the wiring within the fusebox splits, one side going to the mirrors, the other to the HRW.
(the connectors are either side of the screen correct?).
Bottom corners either side.
Finally (sorry!) I changed the stereo today and took the side panels off, someone has been messing in there in the past and I found two wires, 2 core I think, coiled up under the stereo, with a little square plug, one red and one black... These wouldn't be the mirror heating wires? I wouldn't have thought they would be loose and not within a harness but no issues in checking.
That's nowhere near. Must be something else. The mirror wires exit the fusebox and go to either side of the car in the main dashboard loom ( a very thick bundle of wires that runs along the dash crossmember across the full width of the car ).
though is there anywhere under the dash I can check that power is running to the back of the car?
No point at this stage. If the heated mirrors and the HRW aren't working, the fault is almost certainly within the fuse box.